Bumpy Idle after a ride
 

hello. recently we cleaned one of my friends carb and then we bench synced it using a paper. after that we fitted it to the bike and started. it worked really well we let it warm up and set the idle to 1500RPM. engine sound was even and no bumpy idle at all after that we had a ride about 10km. having finished the ride we noticed that the idle was bumpy and we could hear that uneven engine sound very well. it went up and down 300rpm roughly. plugs were working as usual. no cold running cylinders. so we were confused about that. coz this happened only after the ride. what could be the cause ? do i need to use a manometer and balance it ? but it worked very well before going to the ride. please give some advice. thank you.